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Sherlock sends a text – he sets up the setting of the game.
He busies John talking about a non-existent binary code of no use to Moriarty, he pretends it's the key.
But Sherlock Holmes is no amateur, he's figured it all out, and he knows what is going to happen, what Moriarty intends, what the final problem is. And what he's going to do about it.
For now, it's a waiting game.
But when it's time to play, the stakes will be so much higher.
John receives a phone call. Whatever it is, he'll leave.
It'll be any minute now.